Refactor and centralize basic OpenFlow message decoding and validation.

Refactor and centralize basic OpenFlow message decoding and validation.

Open vSwitch contains a few different chunks of code that need to decode
an OpenFlow message to determine its type and then validate that it is
long enough.  Until now, the code for doing this has been more or less
scattered across the tree.  Whenever a new piece of code needed to do this,
it generally needed to reimplement at least part of it.

This commit centralizes all of that work into a single function,
ofputil_decode_msg_type(), and helper functions, and converts all of the
code that was decoding messages by hand to use the new function.